This invention relates to a method of manufacturing quarternary ammonium hydroxide, and more particularly to a method of manufacturing quarternary ammonium hydroxide adapted as a treating agent for the washing of a wafer or the development of a resist layer.
During the manufacture of IC or LSI devices, a treating agent is generally applied, for example, in the washing and etching of the surface of a semiconductor substrate (wafer) and in the development of a resist film. Particularly known among the various treating agents used for the above-mentioned objects, is an organic alkali, such as quarternary ammonium hydroxide, which is free from metal ions, for example, sodium. Particularly in recent years, great demand has been made for quarternary ammonium hydroxide having a high purity and excellent storage stability due to the progress in large-scale integration of semiconductor devices.
To date, quarternary ammonium hydroxide has been manufactured by electrolyzing a salt of quarternary ammonium in an electrolyte cell whose diaphragm is formed of a cation exchange membrane. Said quarternary ammonium salt is provided by a halogenated salt or sulfate ensuring relatively easy synthesis. When, however, it is attempted to manufacture quarternary ammonium hydroxide of great purity by the method involving a halogenated salt, such as an quarternary ammonium salt, the following difficulties are encountered.
(1) The cation exchange membrane has a low capacity for selecting ions and shutting off gases. Consequently, minute amuonts of halogen ions and halogen gases permeate said cation exchange membrane to be carried into the quarternary ammonium hydroxide applied as a cathode solution. When therefore, the quarternary ammonium hydroxide obtained is stored in a widely accepted stainless steal vessel, said vessel is corroded by the highly corrosive halogen ions contained in said quarternary ammonium hydroxide, leading to a decline in the purity of the stored quarternary ammonium hydroxide. Where the quarternary ammonium hydroxide obtained was used as a developer in the manufacture of a semiconductor device, corrosion, etc. of the aluminium interconnection already formed on the semiconductor substrate due to halogen ions, etc. occurred, causing the device to deteriorate.
(2) During the electrolysis of quarternary ammonium salt, highly concentrated halogen ions and halogen gases are generated in an anode solution. As a result, the anode itself formed of a metal such as platinum is corroded by the halogen ions and halogen gases. The products of said corrosion permeate the ion exchange membrane and are carried into a cathode solution, thereby decreasing the purity of the quarternary ammonium hydroxide, and further deteriorating the synthetic resin anode material and cation exchange membrane during the above-mentioned electrolysis. Particularly, the cation exchange membrane, formed of polystyrene, is not applicable at all. Even a cation exchange membrane prepared from highly durable fluorocarbons noticeably deteriorates with time and can not withstand long use.
When a sulfate is applied as a quarternary ammonium salt, not only the drawbacks described in items (1) and (2) are experienced, but also the following problem arises in that difficulties are encountered in handling the extremely harmful alkyl sulfate used as a raw material in the manufacture of the subject quarternary ammonium hydroxide.
In view of the above-mentioned circumstances, patent application disclosure Sho 57-155390 sets forth a method of manufacturing quarternary ammonium hydroxide by electrolyzing quarternary ammonium salt such as acetic tetramethyl ammonium in the aforementioned electrolytic cell. This proposed method can indeed eliminate the difficulties mentioned under items (1) and (2) to manufacture quarternary ammonium hydroxide of high purity. Nevertheless, said disclosed method is still accompanied with the drawback that the quarternary ammonium salt manufactured by said method has a low industrial productivity, that is, a low yield, thereby inevitably increasing the overall cost of the quarternary ammonium hydroxide.